Typically, articles of footwear such as shoes, in particular athletic shoes, may require a certain amount of cushioning to absorb the shock of footstep impact when walking, running or performing other similar activities. In activities such as running or walking, the initial impact of a shoe sole is typically along the outer, lateral edge of the heel of a foot. As the cushioning of the shoe sole heel compresses under the force of the footstep impact, the force of impact may be concentrated on the lateral edge of the heel of a foot and may not be distributed along the sole of the shoe.
In order to provide some amount of cushioning to the wearer of the shoes, midsoles and outsoles are common exterior features of footwear. Generally the midsole of a shoe may comprise a single material or materials of similar densities that are affixed to the bottom surface of a shoe upper. Such midsole configurations may typically be manufactured using traditional polyurethane injection (PU injection) processes.
In regards to polyurethane compounds suitable for injection molding processes, the materials must exhibit certain levels of abrasion resistance, tensile strength, and other such physical properties in order to be fit for use as a shoe midsole. In an effort to improve said physical properties of a midsole, materials having a higher density may be used which may result in an increase in weight.
From a manufacturing standpoint, when using traditional PU injection methods, the addition or inclusion of materials having different densities may result in an increase in cost. The increase in costs may be resultant from an increased number of molds and may further result in an increase in production time. For example, when manufacturing items from injected polyurethane components, such items may require a curing step and each different or distinct material may comprise a different curing method, process, or length of time.
Provided herein, is a method for producing a sole for an article of footwear that allows for two or more densities to be achieved while maintaining the current cost and time of manufacturing relative to other soles. In this way, the overall density of a sole may be reduced which may result in a reduction of overall weight of the article of footwear.
The method briefly described above may allow for a sole to comprise lightweight, low density polyurethane compounds that in other methods may not have been used due to weak physical properties. Specifically, by injecting other higher density polyurethane compounds around the low density polyurethane, a softer or more comfortable underfoot feel may be achieved, while still providing protection and structure needed for durability of the shoe.
In one embodiment of the method, the first layer also referred to herein as the inner compound, may be molded directly onto a strobel board of a shoe or another exterior bottom surface of a shoe upper. In this way, the cushion provided by the low-density inner compound may not be limited by adhesive layers or layers of other bonding agents. The direct molding of the first layer onto the strobel board of a shoe may improve overall cushioning and flexibility of the shoe.
Further, the first layer may be directly molded to the strobel board alone, and the strobel board with the first layer molded thereto may then be stitched to an upper. The upper with the strobel board and first layer may then be placed on an injection machine, wherein a second layer comprising a second material may be injection molded to and around the first layer as well as the strobel board and/or upper. In this way, the process of using injection methods to create a footwear article with a dual-density sole may be simplified, thereby reducing the amount of time to create a footwear article as well as reducing the usage of excess material during the production of the footwear article.
FIGS. 2-3 and 5-12 are drawn approximately to scale. However, other relative dimensions may be used if desired